You said that “Pretty much all the settings are on defaults” including the Email Provider (“No custom provider”), The default provider is just for testing purposes because it has some limitations like:

Sending no more than ten emails per minute.

If you have high bounce rates, the amount of emails you can send may be reduced (or be temporarily blocked). (https://auth0.com/docs/email).

You can fix this setting up your own provider, follow the instructions here: